dm thin: add data block size limits to Documentation

		 $data_block_size $low_water_mark"

		 $data_block_size $low_water_mark"

$data_block_size gives the smallest unit of disk space that can be

$data_block_size gives the smallest unit of disk space that can be

allocated at a time expressed in units of 512-byte sectors.  People

allocated at a time expressed in units of 512-byte sectors.

primarily interested in thin provisioning may want to use a value such

$data_block_size must be between 128 (64KB) and 2097152 (1GB) and a

as 1024 (512KB).  People doing lots of snapshotting may want a smaller value

multiple of 128 (64KB).  $data_block_size cannot be changed after the

such as 128 (64KB).  If you are not zeroing newly-allocated data,

thin-pool is created.  People primarily interested in thin provisioning

a larger $data_block_size in the region of 256000 (128MB) is suggested.

may want to use a value such as 1024 (512KB).  People doing lots of

$data_block_size must be the same for the lifetime of the

snapshotting may want a smaller value such as 128 (64KB).  If you are

metadata device.

not zeroing newly-allocated data, a larger $data_block_size in the

region of 256000 (128MB) is suggested.

$low_water_mark is expressed in blocks of size $data_block_size.  If

$low_water_mark is expressed in blocks of size $data_block_size.  If

free space on the data device drops below this level then a dm event

free space on the data device drops below this level then a dm event